1. Field of the Invention
The present invention relates to detecting redirection or interception of data and in particular to a method of detecting a redirecting process in the course of a bi-directional non-contact making transmission of data.
2. Description of the Related Technology
Systems for the bi-directional non-contact making transmission of data are preferably used in identification systems. In general, these systems consist of a base station and a transponder. These systems are utilised for authentication purposes in the field of motor cars, a main field of usage. In order to achieve a high level of security for the authentication process, the distance over which the communication can take place is restricted to just a few meters in the case of a so-called “passive entry” system, i.e. the opening of a vehicle by pulling the door handle. In an identification system, it is important that the time required for the authentication process be kept short. The total time for the authentication process in the field of motor vehicles is generally between 50 and 130 msec. In order to prevent unauthorised authentication by means of a redirecting process for example, methods have been developed for detecting the manipulation and for terminating the authentication process should this be necessary.
A first method of detecting redirection or interception during an authentication process is known from German patent document DE 10005503, wherein at least one characteristic parameter of the transmitted electromagnetic wave is altered in reversible manner. To this end, a reply signal, which, for example, has been altered in frequency relative to the interrogation signal in a second transmitting and receiving unit (a transponder), is transmitted back to the first transmitting and receiving unit.
In the first transmitting and receiving unit (the base station), the frequency of the reply signal is changed back again and compared with the frequency of the originally transmitted interrogation signal. If the value detected thereby lies within a pre-defined interval, one can virtually exclude the possibility that redirection has occurred.
Another method of detecting redirection in the course of an authentication process is known from German patent document DE 198 27 722. In order to prevent unauthorised opening of a motor vehicle, the power of the transmitted interrogation signal and that of the reply signal are bit-modulated. The mask used for the modulation process is produced by means of a secret key which is known to both the base station and the transponder. A maximum permissible time period is laid down, this being based on the assumption that the modulation of the transmitted power would have to be evaluated in the event of a redirecting process and that an additional time delay would thereby ensue between the transmission of the interrogation signal and the reception of the reply signal. If the time difference between the interrogation and the reply signals is greater than the redefined minimum time, then it is assumed that redirection has occurred and the authentication process is terminated.
The disadvantage of the previous methods is that redirection is not impeded or is not made difficult enough when using the previous methods. It is true that the reversible alteration of the frequency makes the frequency conversion process that is generally carried out during a redirecting process more difficult, but the degree of difficulty involved is determined only by the precision of the frequency conversion process within the redirecting device. Insofar as it is possible to effect a high precision conversion and re-conversion of the frequency in the redirecting devices, then an authentication process can be carried out and unauthorised access to a motor vehicle for example can be obtained. In the case of the other known method, which attempts to detect redirection by encoding the modulation of the transmitter power, this can already be done by the currently known devices (transceivers) that are used for redirecting purposes. Thus the known transceivers compensate for the additional attenuation losses, which are caused by the greater length of the signal path during the redirecting process, by subjecting the signals to linear amplification without thereby altering the relative modulation of the transmitter power. However, as the modulation of the transmitter power does not have to be decoded, the time loss postulated by the method does not occur and redirection cannot be detected. Neither of the two methods offers sufficient protection from unauthorised access within an authentication process.